Former head of Petrobras unit accused him of arranging campaign contributions for his party in 2012

SÃO PAULO—The fledgling administration of Brazil’s acting President Michel Temer was rocked by fresh corruption allegations on Wednesday following the release of plea-bargain testimony implicating him in a sprawling graft scandal centered on the country’s state oil company.

Mr. Temer, who replaced Dilma Rousseff in May when she stepped aside to face an impeachment trial, allegedly asked the then-head of the transportation unit of Petróleo Brasileiro SA in 2012 to arrange illegal campaign contributions to Mr. Temer’s party,...